The manufactuerers didn't care, mileage wasn't such a consideration for many years, not as much as getting more power out the engine was concerned anyway. Then the gas crunch hit and everyone went to smaller motors that were supposed to get better milage. Once the crunch was over it was about getting the most oomph while holding to emissions standards which choked the engines to death & lowered thier economy in several cases. Sure there are several overseas companies that made much more efficient engines in the same period, but in this country fuel resources were not a problem so it was about power, not savings.
If you wanted a better engine and weren't opposed to swapping in a newer block (perish the thought!) you can get very powerful yet economical engines to drop in these days. The GM VorTech motors are an overall good engine for that task.
As for methods to improve what you have, there are loads! Rear end gears are a big one, refreshing or totaly rebuilding a motor helps a lot too. The reality is when trying to make an economical engine there are many crossovers with making it more powerful as well. Stay away from max power and look at what makes it burn the fuel better.
Get friendly with your local machinist, you'll be seeing him alot,...
-Biggest fast improvement is strengthening the components where needed (forged vs. cast pistons) and upping your compression. Compression is so low in our old bolts that you lose a lot of potential power from modern fuels. Bump that 7.5:1 up to 8.5:1 and run higher octane, see what that gets ya. Get it up around 9 and your really cookin with gas!
Keep in mind this will burn hotter as well, better have the cooling system up to snuff, pretty much a requirment anytime you are looking to boost the performance of an engine.
-IF they are available, changing your cam profile can have a huge effect on power output when combined with other mods, it can also make a serious dent in your fuel consumption. Searching around may turn up folks that are making custom grinds and can tailor one to your engine & needs.
-Modifying your exhaust to fit your driving style will make a big change & I have done this with varying levels of success on several vehicles. Smaller diamter pipes for low RPM, low end power, and larger pipes for highway driving & high end torque (this is where folks with big pipes see MUCH better mileage at speed than in town).
-Right along with the exhaust, intakes and carbs need to be modified to breathe evenly in as well as out and is often what messes up peoples performance when they only change one side of the engine. Port matching the manifolds and head & polishing is not that difficult (may be able to goto larger diamter valves as well), allowing an engine to breathe better. Better for power numbers, but can also assist fuel econmy. A carb thats can be easily tuned to fit your engine is a great thing, is a big reason why Webers work wonders. For the best performance overall, you really cant beat a closed loop fuel injection system. But if you're looking to keep the motor stock looking, your options are pretty limited. Altho, there are a couple aftermarket companies that are hiding TBI's in housings that look like stock Carter 2bbl carbs.
Ignition is another one, timing timing timing! EFI conversions are nice, but being able to tune the timing advance is key. Changing your timing curve can make a lot of difference, but you have to be careful there.
If you wanna see some basic power numbers and can get ahold of it, Desktop Dyno is a simulation program that allows you to change things around to see what change has what effect. I believe they have a few Inline-6 profiles in there, nevermind the loads of V8's. I haven't seen any of the recent versions, might be more useful now for making an all around performer than before, used to be more about how to get the most oomph out of your engine. Still, a pretty neat program just to tinker with, but also an effective tool for pre-planning. Then it's about research and $$ (always the $$, gotta love it!).
